Field of the Invention
The present invention relates to a scour prevention material for an underwater structure. More particularly, the present invention relates to a scour prevention material for preventing a structure, such as a bridge or a revetment, provided in a river or a sea shore, in particular, a riverbed or the bottom of the sea under the water surface, near a bridge pier in the river or the sea, from being scoured by flow of water, wave, or the like, when the underwater structure is installed under a rapid tidal stream.
It is well known that when the underwater structure is placed in the bottom of the sea, the structure becomes a resistance against the tidal stream, thus causing swirls, and the ground of the bottom of the sea around the structure is scooped out, with the result that a so-called scour phenomenon occurs which may cause the structure to fall down.
Further, the scour phenomenon is observed for bridge piers of bridges mounted in the river or the sea, and the same phenomenon is also observed with respect to corrosion of the ground of the underwater structure, such as a bank of a sea shore or river.
Even if the location to be scoured is piled up with gravel in advance, or the part that has been scoured is filled with gravel, the gravel is washed away by the water stream, and removed without providing any effect.
Conventionally, various methods for preventing scour have been employed, for example, gabions filed with small stones and crushed stones are placed on the bottom portion near the underwater structure, or a large amount of concrete is deposited, or the structure is embedded in the ground to a greater depth. For the revetment, too, such methods as depositing concrete or using wave suppressing blocks, such as Tetrapods, have been employed.
With the conventional scour prevention method mentioned above, the scour cannot be prevented to a satisfactory level. In the case where a widely used iron gabion is employed, in particular, the gabion tends to get out of shape in a short period of time due to corrosion in the sea or river, and the stones may come out of the gabion. Further, since the gabion is made of iron, it is impossible to flexibly change the shape so that the gabion is closely attached to the structure to be protected, with the result that a large gap or clearance arises between the gabion and the structure. As a result, the portion between the gabion and the structure has an increased coefficient of water conveyance, and thus suffers from a scour phenomenon.
Further, the conventional scour prevention method requires underwater work at the bottom of the sea or river, which is hard to be done under rapid water stream, and it is necessary to reinforce a wide range or area of the ground around the structure. For the Tetrapods or gabions to be sunk to a sufficient depth in the ground during the underwater work, sand and gravel are scattered and placed over the bottom surface of the sea, or the like. Where ready-made articles, such as Tetrapods, are placed under rapid water stream, however, delicate works or operations suitable for the construction field cannot be performed, resulting in undesirable increases in the time, labor and the cost.